Add Merkaartor as submodule

Merkopolo depends on an old version of Merkaartor. That particular Merkaartor version does not compile on Unix due to a missing #include <unistd.h>. Since Merkaartor is not actively maintained anymore I patched the old version with the missing #include to my personal clone of Merkaartor and linked the Merkopolo submodule to this Merkaartor clone.